Recently, in view of the environmental problem and energy saving issues, expectations for a hybrid vehicle and an electric vehicle increase. Thus, it is requested to obtain an electric storage device for storing high energy, which is used repeatedly by charging and discharging electricity.
A lithium ion secondary battery is a promising electric storage device. However, a storage energy density in the battery is limited because of the structure of the battery. Another electric storage device, which may show a storage energy density higher than the lithium ion secondary battery, is a metal air battery.
The metal air battery includes a negative electrode for adsorbing and discharging a metal ion and a positive electrode used as active material of oxygen in air. The oxygen in the outside air is supplied to the battery, so that it is not necessary to accommodate the active material in the battery. Thus, the battery may show the high energy density per unit volume. One of the metal air batteries is a lithium air secondary battery having a negative electrode made of metal lithium. For example, JP-A-2003-7357 teaches the lithium air secondary battery.
JP-A-2003-7357 teaches a concrete construction of the non-aqueous type lithium air secondary battery. Further, JP-A-2003-7357 suggests a magnesium air secondary battery including material as a negative electrode active material capable of adsorbing and discharging a magnesium ion and magnesium metal salt as a support salt.
However, JP-A-2003-7357 merely suggests a possibility of the non-aqueous type magnesium air secondary battery. Thus, JP-A-2003-7357 does not describe a concrete, construction of the magnesium air secondary battery. Actually, when the present inventors manufacture a magnesium air battery including a negative electrode with metal magnesium as a negative electrode active material and a positive electrode with oxygen as a positive electrode active material, the battery merely functions as a primary battery, so that the battery is not charged although the battery can discharge.
It is necessary to resolve magnesium oxide into a magnesium ion and oxygen by electrochemical reaction when the battery is charged. The magnesium oxide is produced at the positive electrode when the battery discharges. However, in general, since the magnesium oxide (MgO) as mainly produced oxide is not easily decomposed, the battery does not function as the secondary battery.